75 to 50 ohm mismatch into a spectrum analyzer

Discussion in 'General Technical Questions and Answers' started by N2NJO, Oct 2, 2008.

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  1. N2NJO

    N2NJO Ham Member QRZ Page

    Measuring 75 ohm signals (off air TV stations for example) from a antenna into a SLM that was designed for 75 ohm vs doing the same into a spectrum analyzer that was designed for 50 ohms.

    Tektronix RFM151 Signal Scout SLM & Instek GSP-827 Spectrum Analyzer

    1. That 75 to 50 ohm 'mismatch' equates to a 1.5:1 mismatch, correct?
    2. If so, is that really a big deal if all I was comparing was signal levels and how the two devices related with the same signal level? A 1.5:1 SWR has always been aceptable as far as I know, so why would it be a issue here?

    The reason for the question, the manufacture of the SA is claiming this is what accounts for the drop in sensistivity between the two devices. The SLM shows a signal 8-10db above the noise floor, while the Instek barely regesters the signal at all.

  2. KL7AJ

    KL7AJ Ham Member QRZ Page


    Now you know
  3. N2NJO

    N2NJO Ham Member QRZ Page

    Ok, then why are they claiming that this is the reason for the difference? There is a 50 to 75 ohm software conversion factor built into the SM that can be used if you are using one of those '75 to 50 ohm pads'.
  4. K9STH

    K9STH Ham Member QRZ Page

    I looked up the data sheets on the two units. Tektronix gives a basic sensitivity of their unit as guaranteed -60 dBm with -65 dBm being typical. On the other hand Instek does NOT list any basic sensitivity.

    Methinks that the Tektronix unit just has a better sensitivity!

    Also, the Instek unit lists the input VSWR as 1.5:1 at 50 ohms which, at least to me, means the input impedance is 75 ohms!

    Glen, K9STH
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    ::I agree. The Tek is just more sensitive, it has nothing to do with impedance matching. You could buy or build a wideband VHF-UHF 50:75 Ohm unun (unbalanced to unbalanced transformer) that has virtually zero loss if you want to convince yourself.

  6. AG3Y

    AG3Y Guest

    It would be interesting to put in a 100 microvolt signal, set the level, and then see what the noise floor would be on each unit. My bet would be that the Tek is a far quieter instrument!

    As stated above, if all you are interested in is relative signal levels, there should really be no issue with that small a mismatch.

    However, you didn't tell us what kind of terminals are on each unit. Remember, you cannot mix 50 and 75 ohm versions of various connectors such as the N style. The pin diameters are different, and trying to put a 50 ohm male connector into a 75 ohm female socket is bound to break a rather expensive connector !

    73, Jim
  7. AI3V

    AI3V Ham Member QRZ Page

    Check the resoloution bandwidth, and video bandwidth of both instruments,I think you will find the instec resoloution bandwidth is set wider than on your Tektronix unit.- by a factor of ten.

    The wider resoloution bandwith lets more noise thru.That makes the unit appear less sensitive.

    Notice that the trace is above the bottom of the display.

    For a accurate idea of the actuall c+n/n you want Both units set to the same bandwidth as the receiver this signal feeds.

    The difference between 50 and 75 ohms is mostly accademic in this case, and I wold not worry about it.

    Last edited: Oct 2, 2008
  8. AG3Y

    AG3Y Guest

    Rege is absolutely correct ! Ding ding !

    Obviously, if you slow down the sweep rate and get higher resolution, your noise floor will drop significantly! You must make sure that the resolutions, sweep rates, etc. are all matched, or else you are just comparing apples and oranges!

    Really ignorant of me to overlook the parameters that you had set up on the two units. Nice catch, Rege !
  9. AI3V

    AI3V Ham Member QRZ Page

    Thanks Jim!

    I have danced with this girl before. :D

  10. KL7AJ

    KL7AJ Ham Member QRZ Page

    Or on the other end....inserting a 75 ohm male N into a 50 ohm female N is likely to result in NO CONNECTION between the center conductors! This drove us bananas for about a day when we were doing a TV antenna proof. We had just enough capacitive coupling to act like a high-pass filter!

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